DSSS networks are used in implementing over the air systems. In one example, a smart utility network (SUN) is a low rate (5 kb/s to 1 Mb/s), low power, wireless communications technology that is specifically designed to be used in utility metering applications, such as for transmitting electric, gas, or water usage data from the one or more meters on the customer premises to a data collection point operated for the utility. The data collection point can then be connected to a central office for the utility by a similar or a different interface, which can be a high speed “backhaul” such as an optical fiber, copper wire, or other high speed wired connection to a network including the central office.
In the prior known solutions, different physical layers (PHYs) can be used for communication in over the air networks such as SUN including frequency shift keying (FSK), direct sequence spread spectrum (DSSS), and orthogonal frequency division multiplexing (OFDM). In an example DSSS communications system that is a closed utility network, the devices that are allowed into the network can be controlled by the utility or the network operator. Note that while some of the examples discussed herein for illustration include the operation of smart utility networks, the arrangements disclosed as aspects of the present application are not so limited and can be applied and used in conjunction with DSSS communications networks, generally.
A relevant standard has been promulgated by the IEEE, referred to as IEEE standard number 802.15.4g, entitled “Low-Rate Wireless Personal Area Networks (LR-WPANs)” issued Apr. 27, 2012 by the IEEE Computer Society and sponsored by the LAN/MAN Standards Committee. This standard identifies physical layer (PHY) specifications for low data rate, wireless, smart metering utility networks (SUN). The standard is intended to provide a globally used standard that facilitates very large scale process control applications such as a utility smart-grid network that are capable of supporting large, geographically diverse networks with minimal infrastructure and containing potentially millions of fixed endpoints. Note that arrangements of the present application is not limited to particular environment, including the SUN applications, but the various arrangements that form aspects of the present application are applicable to such applications.
FIG. 1 is an illustration of a portion of a 100 kchip/sec DSSS physical layer packet as defined by the IEEE 802.15.4g specification. In a DSSS communication system, a “chip” refers to a single electrical pulse with duration equal to 1/chip second rate. For example, in a 100 Kchip rate system, the chip duration would be 1/100,000=10 uS. The DSSS packet 100 in FIG. 1 includes fields such as a synchronization header (SHR) 110, physical layer header (PHR) 112 and a physical layer service data unit (PSDU) 114. Each SHR contains a preamble 120 and a start frame delimiter (SFD) 122 that can be utilized by the receiver for detecting the DSSS packet. For the 100 kchip/s mode, the preamble 120 contains 32 bits that are spread with a spreading code of 32 to form a string of 1024 chips (32 bits×32 spread) as indicated 130, 132. The SFD 122 contains 16 bits that are also spread with spreading code of 32. An aspect of the current application provides methods and apparatus that will improve the detection of the preamble 120.
In the example communications systems described herein, phase shift keying is used to modulate data. In phase shift keying (PSK), digital modulation is used to transmit data by modulating the phase of a reference signal or carrier. In quadrature phase shift keying (QPSK), four points on a constellation diagram are used that are equispaced around a circle, providing four phases. Accordingly 2 bits can be used to indicate the quadrature phase. In the examples described here, offset QPSK or O-QPSK modulation is used. In O-QPSK, the two portions, the in-phase and quadrature components or I and Q components of a symbol, are transmitted with an offset between them. In this fashion only one bit of a two bit coding scheme changes at a given time and thus reception errors that could occur in the reception of the symbols due to noise or interference can be reduced.
FIG. 2 is a block diagram depicting a standardized transmitter of the prior art for DSSS packets, as defined by the IEEE 802.15.4g specification. The chip timing for the Offset Quadrature Phase Shift Key (O-QPSK) modulator 210 is shown in FIG. 1. Transmitted data is coded, spread and concatenated at 202 before being coupled to the O-QPSK modulator 210. The O-QPSK modulator 210 contains a local oscillator 282 that is designed to oscillate at the same prescribed frequency as the receiver's local oscillator and serves to generate the transmitter's carrier frequency. The receiver has the same frequency for demodulation of received signals.
FIG. 3 is a block diagram depicting a prior art system 300 performing the detection of DSSS preambles using differential sequences. In FIG. 3, the receiver/sampler 302 is coupled to the O-QPSK demodulator 380. Within the demodulator 380, a local oscillator 382 is designed to oscillate at the same prescribed frequency as the transmitter's local oscillator (shown in FIG. 2, above). The O-QPSK demodulator 380 is coupled to, for example, a chip differential block 312 which is coupled to an accumulation block 314. The accumulation block 314 is coupled to a correlation block 316 which produces an output 318, indicating that the preamble has been found. The flow 312-316 is illustrated for the first sample. Similar processing occurs for each subsequent timing signal as they arrive as illustrated by the 3 flows 322-326 for 2nd sample, 332-336 for 3rd sample and 342-346 for 4th sample.
In this example 300, 4× over sampling is used to sample the signal in block 302 at 4 discrete times. The 4 signal samples arrive sequentially at the rate of one sample per oversample clock. The processing of each of the signal samples is illustrated by the series of 3 blocks 312, 314, 316. The processing in each of the blocks (312, 314 and 316) is completed within 1 oversampled clock period. Although the processing flow for each of the 4 timing samples is depicted in 300, in the actual receiver, there may only be a single instance of each block since the sampled signal arrives and is processed sequentially through the blocks. Alternatively, more blocks can be used.
In operation, as shown in FIG. 3, a DSSS signal is received from an antenna, for example, and sampled at 4× the chip rate in block 302. The I/Q signals 303 are passed to the O-QPSK demodulator 380 where the I/Q signals are down-converted utilizing the receiver's local oscillator 382 as the carrier frequency. The differential chip values for the first phase are calculated in block 312 and accumulated together with prior data in block 314. After accumulation, each sample can be correlated in block 316 to a known chip spreading value and the result can be compared to a threshold value. When the threshold value is exceeded, that result indicates that a preamble has been located (in block 318). In one approach a single signal sample can be used to detect a preamble location in the sequence, however, processing of additional signal samples 322-342 improves the success of preamble detection and improves the performance of the system by detecting the preamble using the corrected carrier frequency. The use of the arrangements can enable preamble detection even in environments with low signal to noise (SNR) characteristics, including environments with interfering devices.
An example DSSS signal preamble detection system is described in U.S. Patent Application Publication No. US 2013/0202014A1, entitled “DSSS Preamble Detection for Smart Utility Networks,” to Timothy Mark Schmidl, which is co-owned with the present application, and which application is hereby incorporated in its entirety herein by reference.
The DSSS receivers can be used in a utility network. For example, utility meters are typically installed for each house in a residential neighborhood. The meter data can be transmitted wirelessly via a RF transmission and be received and read by either a mobile or fixed point collection device called a reader. Some meters will broadcast periodically so the reader is simply a receiver. In other instances, the customer premises meter's transmitter remains in a sleeping state waiting for a transmission or poll message from the reader. The reader's transmission makes the meter “wake up” and transmit the accumulated data which is then received by the reader.
Mobile readers can be vehicle mounted or carried in person by a meter reader or otherwise deployed. For the safest and fastest data collection, the mobile readers prefer to stay on public thoroughfares, so that there is no need to enter the customer's property. This allows utility meter data collection under circumstances where the meter cannot be seen, when the customer is not home, the property has dangerous conditions such as construction or dogs present or some other impediment that makes physically reading the meter data difficult. The RF meter data collection occurs by maneuvering the mobile reader close enough to the meter to enable the signal to be read. At the end of a route or meter reading session, the mobile reader returns to a central office or base location and the meter readings are transferred for processing.
Mobile readers are very common and a meter reader simply checks out a reader prior to collecting data on a route. Because the meters are used in this fashion, there is an opportunity for a different physical reader being used along a given route at different reading times. Each of the readers includes a DSSS receiver that performs the preamble detection process as described above, including having a different a local oscillator which can be oscillating at a different frequency from the prior readers used to read the meters on a route.
A characteristic feature of utility networks is that the customer meters often have a long life span, such as 20 years, and across an established utility area there are typically many generations of customer meters deployed. Within these meters, the local oscillators are initially designed to have the same frequency, however because of component tolerances, temperature, aging and other imperfections, the local oscillator frequency of the transmitters can drift from the intended frequency resulting in a large carrier frequency offset between the transmitter/receiver pair. This carrier frequency offset characteristic makes preamble detection more difficult for the DSSS receiver by causing a continuous rotation in the signal constellation, thus slowing down the process of reading meters sometime requiring either multiple passes, entering the customer premises to obtain the meter reading and eventually replacing the meter. The preamble detection can also be performed in a low signal to noise ratio environment, where the signal is compromised by various interference or noise sources. Carrier frequency offset in a low SNR environment can cause prior known approaches to preamble detection to fail or to cause the need for repeated attempts. While these problems are presented in a utility meter example above, carrier frequency offset between receivers and transmitters can occur in other DSSS communications systems and the arrangements of the present application apply to DSSS systems generally, as well as to SUN applications. Generally, the arrangements of the present application improve performance in DSSS signaling systems where carrier frequency offset can occur between a transmitter and a receiver.
FIG. 4 depicts in a block diagram a communications network 400 with a fixed receiver 482 with several smart meters and their local oscillators. In FIG. 4, a utility 410, such as gas, water or electricity, is available through an area where customers are provided with smart meters (420, 430, 440, 450) each having the capability to measure the consumption of the utility, and to transmit that information using the DSSS coding and a local oscillator (422, 432, 442, 452) set to a prescribed frequency. A fixed meter reader 480 is depicted with a DSSS receiver and local oscillator 482 set to a prescribed frequency. The reader 480 is connected to a link 412 which transfers collected data back to a billing center or central office. The link 412 could be implemented as copper, fiber, or wireless method and can be referred to as a “backhaul.” Fixed readers 480 are typically mounted on pole tops and can be connected to the main office by various methods including copper, fiber optics or wirelessly 412. Fixed readers offer the utility lower recurring costs since the meter readings occur automatically without the need for an employee to either walk or drive the route to collect readings. Imperfections in the local oscillators in the devices (422, 432, 442, 452 and 482) cause a carrier frequency offset. The carrier frequency offset results in difficulty for the DSSS receiver in the fixed meter 480 to detect the preamble needed receive the DSSS header sent by the smart meters (420, 430, 440, 450). This degradation in preamble detection can reduce the number of meters a fixed reader can service in a fixed period. In other situations, the data may have to be collected by a mobile reader placed closer to the customer premises, causing additional expense.
Improvements in the reception of DSSS signals having carrier frequency offset, or reduction of carrier frequency offset in received signals which will improve the preamble detection in DSSS systems, are therefore needed in order to address the deficiencies and the disadvantages of the prior known approaches. In the SUN application, improvements which will extend the utility meters useable lifetime and improve a meter reader's ability to capture meter data are desirable.